1. Field of the Invention
This invention is embodied in a novel process for ethynylating aromatic compounds, which yields a series of novel ethynylated compounds and also provided a more efficient method for the production of certain prior art ethynylated compounds.
Compounds prepared by the process described and claimed below are suitable for use in the synthesis of various acetylene-terminated benzimidazoles and acetylene-terminated benzimidazoquinazolines that are useful as high temperature structural resins.
In addition to facilitating the preparation of novel acetylene-substituted aromatic compounds, the process of this invention provides an improved method for the synthesis of aminophenylacetylenes, hydroxyphenylacetylenes and halogenated phenylacetylenes which are important intermediates in acetylene-terminated polyimide and polyimide oligomers and polymers.
2. Description of the Prior Art
An example of prior art, known to applicants, and possibly relevant to this invention is described in an article entitled the "Synthesis of Ortho-Ethynylbenzaldehyde and an Approach to Dibenzoannulene" published by Juro Ojima, ET AL of Toyama University in Gufeku, Toyama, Japan in Chemistry Letters at pp. 633-636 in 1972. The process to prepare ortho-ethynylbenzaldehyde disclosed in the Toyama publication is a relatively inefficient multiple step process which provides relative low yields and is costly both in materials and in time consumed in preparing this compound.
A widely accepted procedure for the introduction of an acetylenic substituent onto an aromatic nucleus is known as the Stephens-Castro coupling reaction between an aryl iodide and a stoichiometric amount of a protected cuprous acetylide and pyridine at reflux. See C. E. Castro and R. D. Stephens, J. Org. Chem. 28, 2163 (1963). Commonly used protecting groups for acetylenes are acetals, ketones, ketals, hydroxymethyl and tetrahydrofuran-protected hydroxymethyl, dimethylcarbinol, ethyl vinyl ether-protected carbino, and ethyl ester. The removal of these groups often requires several steps and strongly alkaline media.
An elegant alternative to the above Stephens-Castro method rests in the coupling between arylcopper reagents and iodoethynyltrimethylsilane at below ambient temperatures. The removal of the trimethylsilyl group with weak alkaline is quantitative. However, the in situ preparation of the arylcopper reagents depends on prior formation of Grignard or lithium reagents, a requirement that forbids the presence of functional groups which are incompatible with the organometallic reagents.
One of the prior art methods most relevant to the present invention for preparing ethynalated compounds, and known to Applicants herein, is known as the Heck reaction, [R. F. Heck, Accts of Chem. Res., Vol. 12, 146 (1979)] which is a reaction between phenylacetylene and aromatic halides in the presence of an organopalladium catalyst. A parallel procedure in effecting ethynylation of aromatic compounds employs the palladium catalyzed coupling of aromatic bromides and iodides with 2-methyl-3-butyn-2-ol. See C. A. Onopchenko, E. T. Sabourin, and C. M. Selwitz, J. Org. Chem, 44, 1233 (1979); E. T. Sabourin and C. M. Selwitz, U.S. Pat. No. 4,128,588. Dec. 5, 1978; A. Onopchenko, E. T. Sabourin, and C. M. Selwitz, U.S. Pat. No. 4,139,561. Feb. 13, 1979; E. T. Sabourin and C. M. Selwitz, U.S. Pat. No. 4,210,610. July 1, 1980; E. T. Sabourin and C. M. Selwitz, U.S. Pat. No. 4,223,172. Sept. 16, 1980; and also B. A. Reinhardt and F. E. Arnold, United States Airforce Wright Aeronautical Laboratories Technical Report, AFWAL-TR-80-4012, April, 1980. In all of the above publications, the removal of the acetonyl protecting groups requires sodium hydroxide in refluxing toluene. Therefore, base-sensitive groups cannot be present.
An entirely different process for the synthesis of aminophenylacetylenes is found in U.S. Pat. No. 3,928,450 issued to Norman Bilow on Dec. 23, 1975 and assigned to the present assignee. While the process disclosed in the above '450 Bilow patent is effective and serves its intended purpose, it also requires corrosive reactants and is difficult to operate, thereby resulting in an exceedingly expensive product.